Abstract: A brush cassette is detachably loaded to an endoscope washer disinfector for washing and disinfecting an endoscopic duct of an endoscope. The brush cassette comprises a washing brush, a chamber accommodating the washing brush, and a brush delivery duct that allows the washing brush to pass therethrough toward the duct of the endoscope. Further the brush cassette comprises a fluid supply duct that allows a chemical to pass therethrough toward the chamber; a drive member used for moving back and forth the washing brush between the chamber and the duct via the brush delivery duct; a means that prevents air from accumulating in the chamber; and a remaining-fluid discharge duct that allows the chemical remaining in the chamber to be discharged outside therethrough. The preventing means is a duct discharging the air or a deflection member deflecting is the chemical toward a region in which the air is to accumulate.